JP2679839B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that irradiates a photosensitive member with a beam based on the value of input digital multi-valued data to form a visible image. Is.

[Prior Art] A typical example of this type of device is a laser beam printer.

Usually, in this apparatus, a pulse width modulation (PWM) method or a dither method is often adopted as a method for reproducing halftone.

The PWM method is a pulse signal with a pulse width corresponding to the value of the input multi-valued data by converting the input multi-valued data into an analog signal once and comparing it with a periodic pattern signal such as a triangular wave. Is what you get. The pulse width modulated signal is used to control the irradiation time of the laser beam for one pixel, for example, to express the gradation. However, since this PWM method is processed in units of one dot, there is a problem in that the resolution is high but sufficient gradation cannot be obtained.

Further, in the case of the dither method, if the threshold matrix is increased, the gradation can be enhanced, but on the contrary, there is a problem that the resolution is remarkably lowered.

[Problems to be Solved by the Invention] If an output image is taken as an example, when printing an original in which characters and photographs are mixed, characters are blurred when trying to enhance the gradation of the photographs. Moreover, if the characters are to be sharpened, the gradation of the photograph will be degraded.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image processing apparatus having excellent resolution and gradation.

[Means for Solving the Problem] The present invention for solving this problem includes the following configurations. That is, in an image forming apparatus that irradiates a photoconductor with a beam based on the value of input digital multi-valued data to form a visible image, the input m-bit digital multi-valued data is converted into n, m-n.
Separation means for separating bit data, first conversion means for converting the separated n-bit data into a pulse width modulated signal, and m-n bit data separated by the separation means based on predetermined matrix data It comprises a second converting means for converting into a binarized signal, and a combining means for combining the signals converted by the first converting means and the second converting means.

[Operation] In the structure of the present invention, the separating means separates the digital multilevel data, and the separated n bits are converted into a pulse width modulation signal. And the remaining m
For the -n bit, it is converted into a binary signal based on predetermined matrix data. Then, these converted signals are combined, and a beam based on the combined signal is applied to the photoconductor to form a visible image.

Embodiment An embodiment according to the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram of an image signal processing system in the laser beam printer of this embodiment.

In the figure, reference numeral 1 denotes a γROM for γ-correcting the inputted 8-bit (256 gradations) multivalued data, which constitutes a so-called lookup table. This γROM1 is provided to correct the input density data in consideration of the output characteristics of this printer. The conversion curve of γROM1 is as shown in Fig. 2, and the density data is input to the address line of γROM1,
The data written at that address is output from the data line. For example, "AO _H (H
Is a hexadecimal number) ", the data of" 90 _H "is output.

Reference numeral 5 denotes an image clock generator, which generates a signal obtained by dividing the clock from the oscillator 7 by 4 as an image clock 5a. The data of γROM1 is output in synchronization with this image clock 5a.

 The other components will be described in order.

The corrected pixel data 1a output from the .gamma.ROM1 is separated into the upper 2 bits and the lower 6 bits constituting the data.

Most significant 2 bits is input to the D / A converter 2 is converted into an analog signal 2a of 4 levels _{(O H, 40 H, 80} H, corresponding to CO _H). The comparator 4 compares the sawtooth wave 3a generated by the sawtooth wave generator 3 (generated in synchronization with the image clock 5a) to convert the signal in the depth direction into a signal in the length direction (pulse). Width modulation), and the modulated image signal 4a is output to one input side of the OR circuit 10.

 The state of pulse width modulation will be described with reference to FIG.

As shown in the figure, the four-level (level 0 to 3) analog signal 2a is converted into an image signal 4a by comparing the sawtooth wave 3a synchronized with the image clock 5a. here,
The generated image signal 4a has a width of 0/4 to 3/4 when one dot width of the image clock 5a is "1".
It can be seen that the signal has a width (corresponding to levels 0 to 3).

On the other hand, the lower 6-bit data out of the 8 bits output from γROM1 is input to the dither generator 6 and subjected to dither processing.

An 8 × 8 threshold matrix is held inside the dither generator 6, and one of these thresholds is compared with the input 6-bit data in synchronization with the image clock 5a to output the dither signal 6a.

 This is shown in FIG.

FIG. 4A shows the input 6-bit image data, and FIG. 4B shows the selected threshold value (or the threshold value in the threshold value matrix). Then, the value of the image data is compared with the corresponding threshold value, and when the image data is larger, it is output as black, and when it is smaller, it is output as white. In the case of FIGS. 4 (a) and 4 (b), FIG. 4 (c) is the image signal 6a after dither processing. However, in the figure, the shaded portion indicates black.

Now, the image signal 6a output from this dither generator 6
Is then input to the AND circuit 9. And this AND circuit 9
Is a logical product of the input image signal 6a and the timing signal 8a from the timing counter 8
Output to 10. Thereby, the output timing of the image signal 6a is controlled.

 This will be described with reference to FIG.

The dither generator 6 performs the above-mentioned dither processing in synchronization with the image clock 5a and outputs the image signal 6a. At this time,
Timing signal from the timing counter 8 as shown
Since 8a is output, the AND circuit 9 outputs the result of dithering each pixel as an image signal 9a. The important point here is the image signal 9a.
Is output as a quarter of the end when the one-dot width of the image clock 5a is "1".

In this way, both the PWM-converted image signal 4a and the dither-processed image signal 9a are input to the OR circuit 10, where they are logically summed and output as the image signal 10a, which is represented by this signal. An example of the image displayed is shown in FIG.

As shown in the figure, when one pixel is divided into four, three masses are based on the pulse width modulation (PWM) method and the remaining one maldither processing. In other words, even _{connexion, O H, 40 H, 80} H, represents the concentration (4 tones) of CO _H, represent one of the dither component of 8 × 8 dots and the remaining one square at 3 mass by PWM ( 0 to 3F _H = 64 gradations).

The image signal 10a is output to a printing system (not shown) and used to turn ON / OFF the light emission of the laser unit.
The description itself is omitted because it is based on a known technique.

<Description of Other Embodiments> FIG. 7 is a block diagram of the second embodiment.

 The same components as those in FIG. 1 are designated by the same symbols.

In the figure, 100 is a ratio control circuit for controlling the weighting ratio of PWM control and dither control. Specifically, whether the resolution is increased to reduce the gradation or the gradation is increased to decrease the resolution is controlled by, for example, a command input from the outside (host computer). Reference numeral 11 is a data division circuit, and according to the ratio indicated by the ratio control means 100,
D / A converter 1 for n bits of 8-bit data of image data 1a
3 and output the remaining 8-n bit data to dither generator 1
Output to 5. Reference numeral 12 denotes a resolution control circuit, which controls the data division circuit 11 so that the resolution corresponds to the number of bits assigned to the D / A converter 13. A dither matrix selection circuit 14 instructs the dither generator 15 to select a dither matrix corresponding to the number of bits assigned to the dither processing by the data division circuit 11.
In response to this instruction, the dither generator 15 selects a dither matrix of a corresponding size from a plurality of dither matrices provided in the dither generator 15 and motivates the image clock to perform dither processing.

An example of a mixed image of the PWM processing and the dither processing in the above configuration is shown in FIGS. 8 (a) and 8 (b).

Explaining FIG. 8 (a), this is the case when the resolution is increased. The upper 4 bits of the 8 bit data from γROM1 are processed by the PWM method and the lower 4 bits are processed by the 4 × 4 dither method. An example is shown. Further, FIG. 8 (b) shows a case where the gradation is increased, one bit for PWM and seven for dither.
An example in which bits are assigned is shown.

As described above, according to this embodiment, a few bits of the input data are subjected to the pulse width modulation processing, and the remaining bits are subjected to the dither processing, so that it is possible to obtain an image excellent in resolution and gradation. Become.

Further, by making variable the weighting of the resolution and the gradation, it is possible to make fine adjustments, which is convenient.

In the embodiment, a sawtooth wave is used as an example for the PWM processing, but the present invention is not limited to this. The point is that it suffices if the ON / OFF signal of the laser can be generated by the signal obtained by both the PWM processing and the dither processing.

Further, the gradation reproduction processing is not limited to the dither method. This is because, for example, the error diffusion method may be used.

Furthermore, in the second embodiment, the weights of the resolution and the gradation are changed by the command from the host computer. However, a switch to that effect is provided in the printer itself, and the control is also performed by the switch. Of course good things.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain an image having excellent resolution and gradation.

Further, if the resolution and the weighting to the gradation can be changed, delicate adjustment can be performed, which is very convenient.

[Brief description of the drawings]

FIG. 1 is a block configuration diagram of an image processing system in the present embodiment, FIG. 2 is a diagram showing conversion characteristics of .gamma.ROM in the present embodiment, FIG. 3 is a diagram for explaining pulse width modulation in the embodiment, and FIG. 4A to 4C are diagrams for explaining the principle of the dither processing in the embodiment, FIG. 5 is a timing chart showing the output timing of the signal after the dither processing, and FIG. 6 is a pulse width in the embodiment. FIG. 7 is a diagram showing an example of a mixed image by modulation processing and dither processing, FIG. 7 is a block diagram of an image processing system in the second embodiment, and FIGS. 8 (a) and 8 (b) are mixed images in the second embodiment. It is a figure which shows an example. In the figure, 1 ... γROM, 2 and 13 ... D / A converter, 3 ... saw generator, 4 ... comparator, 5 ... image clock generator, 6 and 16 ... dither generator, 7 ...... Oscillator, 8 …… Timing counter, 9 …… AND circuit, 10 ・ ・ ・
... OR circuit, 11 ... Data division circuit, 12 ... Resolution control circuit, 14 ... Dither matrix selection circuit.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasutaka Noguchi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Atsushi Kashiwara 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Incorporated (72) Inventor Takashi Kawana 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) Reference JP 61-61566 (JP, A) JP 63-286351 (JP , A)

Claims (2)

(57) [Claims] 1. An image forming apparatus for forming a visible image by irradiating a photosensitive member with a beam based on the value of input digital multi-valued data, and converting the input multi-valued digital data of m bits into n, m-n bits. Separation means for separating the data, first conversion means for converting the separated n-bit data into a pulse width modulated signal, and binary data for the mn bit data separated by the separation means based on predetermined matrix data An image forming apparatus comprising: a second conversion unit that converts the converted signal into a converted signal; and a combining unit that combines the signals converted by the first conversion unit and the second conversion unit. 2. The image forming apparatus according to claim 1, further comprising an adjusting unit that adjusts the ratio separated by the separating unit.